The present invention relates to a method and apparatus for soldering a first plurality of electrical contacts to a second plurality of electrical contacts. The invention has particular utility in joining flexible etched cables, ribbon cables and surface mount connectors to contact pads on printed circuit boards and may also be employed to join two single contacts instead of pluralities of contacts.
Although the invention has its primary utility in simultaneously joining first and second pluralities of contacts, it will be appreciated that the principles described herein are equally applicable to joining a single contact to another single contact. Moreover, although the description set forth below mentions solder as the joining material, it is to be understood that any fusible material, such as doped conductive plastic material, may be employed.
Soldering flat cable leads to circuit board contact pads can be achieved by a variety of prior art methods and apparatus. The least desirable of these is manually effecting each of the multiple solder terminations because the resulting repetitive soldering operations are time-consuming and costly. In addition, close spacing between adjacent connection sites renders it likely that one or more of the manual soldering operations will result in solder bridges between one or more pairs of the adjacent sites.
There have been a number of prior art attempts to solve the aforementioned problems, examples of which may be found in U.S. Pat. Nos. 3,396,894 (Ellis), 3,719,981 (Steitz), 3,750,252 (Landman) and 4,484,704 (Grassauer et al). Typically, in these and other prior art soldering procedures for forming multiple solder joints simultaneously, a soldering tool is employed to deliver the necessary thermal energy over a large continuous area spanning all of the connection sites. Upon energization, the soldering tool heats up until it overshoots a control temperature before settling down to that temperature. The control temperature is typically chosen somewhat above the ideal soldering temperature in order to compensate for less than ideal thermal energy transfer. This approach to thermal energy delivery has a number of disadvantages. One such disadvantage is damage to components resulting from overheating. For example, the thermal overshoot inherent in the heating tool can damage components disposed between the connection sites within the area heated by the tool. In some cases the overshoot may cause damage to the polymeric materials, insulating materials and adhesives at the connection site. It is tempting to suggest that the operator of the soldering tool might avoid the thermal overshoot by either removing the tool before the overshoot occurs or delaying application of the tool until after the overshoot occurs. This is impractical for a number of reasons. First, there is no evident indication as to when the thermal overshoot occurs. Second, although the tool warm-up time is quite long, the time interval during which the tool temperature is sufficient to melt solder, but prior to overshoot, is too short to reliably complete the soldering operation. Further, where the tool is also employed to apply pressure to the connection site, the power must be turned off after the solder melts while pressure is maintained on the tool until the solder solidifies. The tool must be re-energized to effect the next soldering cycle. The repeated on-off cycling changes the starting temperature for the transient overshoot in each cycle, thereby making it virtually impossible to determine when the tool attains the final control temperature.
Another prior art problem associated with the simultaneous soldering of sets of plural contacts relates to solder bridging between connection sites. The pressure and thermal energy applied to spaces between connection sites tends to cause the solder to run between those sites and form solder bridges. This problem has been addressed in some prior art apparatus such as that disclosed in the aforementioned Grassauer et al patent. In that apparatus the solder is sandwiched between two layers of polymeric material, one of which has window openings arranged to permit the solder, when melted, to flow through to respective connection sites. Barriers are provided between the windows to preclude solder bridging While this technique minimizes bridging when properly employed, proper employment is hampered by the difficulty of aligning the windows (which must necessarily face downward and away from the technician) with respective contact pads at the connection sites. Care must also be taken to avoid displacement of the solder within the layered polymeric package prior to heating the apparatus in order to assure that solder is present at each of the window openings.
It is desirable, therefore, to provide a method and apparatus that permits simultaneous soldering at multiple connection sites without applying thermal energy to spaces between those sites and without creating solder bridges between adjacent connection sites. Moreover, the method and apparatus should be equally useful in joining a single contact to another single contact. In addition, it is desirable that the thermal energy required to melt the solder be available virtually instantaneously after energization of the heater, and that the heater be arranged to provide no more thermal energy than is required to melt the solder employed for the various connection sites. Finally, it is desirable that the apparatus have relatively low mass in order that it may cool down quickly after a soldering operation.
The present invention makes use of a relatively new automatic self-regulating heater technology disclosed in U.S. Pat. Nos. 4,256,945 (Carter et al), 4,623,401 (Derbyshire et al), 4,659,912 (Derbyshire), 4,695,713 (Krumme), 4,701,587 (Carter et al), 4,717,814 (Krumme) and 4,745,264 (Carter). The disclosures in these patents are expressly incorporated herein by reference. A heater constructed in accordance with that technology, hereinafter referred to as a self-regulating heater, employs a substrate of copper, copper alloy, or other material of low electrical resistivity, negligible magnetic permeability and high thermal conductivity. A thin layer of thermally-conductive magnetic material is deposited on all or part of one surface of the substrate, the layer material typically being an iron, nickel or nickel-iron alloy, or the like, having a much higher electrical resistance and magnetic permeability than the substrate material. The thickness of the layer is approximately one skin depth, based on the frequency of the energizing current and the permeability and resistance of the layer. A constant amplitude, high frequency alternating energizing current is passed through the heater and, as a result of the skin effect phenomenon, is initially concentrated in one skin depth corresponding to the thickness of the magnetic material. When the temperature at any point along the heater reaches the Curie temperature of the magnetic material, the magnetic permeability of the magnetic material at that point decreases dramatically, thereby significantly increasing the skin depth so that the current density profile expands into the non-magnetic substrate of low resistivity. The overall result is a lower resistance and lesser heat dissipation. If thermal sinks or loads are placed in contact with the heater at different locations along the heater length, thermal energy is transferred to the loads at those locations with the result that the temperature does not rise to the alloy Curie temperature as quickly at those locations as it does in the non-loaded locations. The constant amplitude current remains concentrated in the higher resistance alloy layer at the loaded locations which dissipate considerably more resistive heating energy than is dissipated in the non-load locations where the current is distributed in the low resistance substrate.